Interestingly, I liked the Spacechem the most (probably because the program is represented in the most visual form of all their games, and it has excellent music and it's very calming to watch a resulting solution), although I didn't finish the last optional puzzle and any of the expansions.

I would love to go back and finish those, but what holds me back is that I would have to replay the whole game. All the tricks I learned about how to play it I have already forgotten.